1. Field of the Invention
The present invention pertains to a method and apparatus for plasma cleaning of semiconductor processing chambers.
2. Description of the Background Art
Semiconductor processing involves a number of different chemical and physical processes whereby minute integrated circuits are created on a substrate. The integrated circuits are constructed using multilayers of interrelated patterns of various materials; layers of material are created by chemical vapor deposition, physical vapor deposition, and epitaxial growth. Some of the layers are patterned using photoresist masks and wet and dry etching techniques, Patterns are created within layers by the implanting of dopants at particular locations. (The substrate upon which the integrated circuit is created may be silicon, gallium arsenide, glass, or any other appropriate material). Many of the processes carried out within semiconductor processing reactors leave contaminant deposits on the walls of the process chamber which accumulate and become the source of particulate matter harmful to the creation of a semiconductor device. As the dimension size of the semiconductor device has become ever smaller, the presence of particulate matter upon the surface of the semiconductor workpiece has become ever more critical.
Contaminant deposit buildup on semiconductor process chamber walls can be particularly significant when metal etching processes are carried out in the chamber. In particular, the etching of an aluminum pattern produces relatively large accumulations of such contaminant buildup. For example, during experimental plasma etching of aluminum upon the surface of a semiconductor workpiece, under disadvantageous conditions, etching of 25 ea. 8 in. diameter silicon wafer substrates produced an average contaminant layer thickness of about 1 micron built up on the walls of the etch processing chamber. The contaminant deposit thickness was greatest on the etch chamber walls and gas distribution plate which were in contact with the active plasma; the etch chamber had a diameter of about 15 in. and a height of about 8 in.
Contaminants can be removed from the walls of the processing chamber and the gas distribution plate by dry cleaning using plasma-enhanced etching, or the processing chamber surfaces can be opened and wet cleaned manually. This latter procedure for removing contaminants from the processing chamber wall is very time consuming.
U.S. Pat. No. 5,207,836 to Chang et al., issued May 4 1993, describes a cleaning process for removal of deposits from the susceptor of a chemical vapor deposition apparatus. The process is recommended for the removal of deposits such as tungsten or tungsten silicide from a susceptor in a vacuum deposition chamber. To avoid leaving fluorine residues in the deposition chamber, after a gaseous source of fluorine is used in the plasma cleaning of the chamber, a gaseous source of hydrogen is fed into the chamber (while the plasma is maintained) to remove any fluorine residues from the chamber. Examples of fluorine-containing gases recommended for the dry cleaning include SF.sub.6, CF.sub.4, C.sub.2 F.sub.6, and NF.sub.3. The gaseous source of fluorine may further include inert or non-reactive gases such as argon, neon, or helium.
U.S. Pat. No. 5,202,291 to Chavrat et al., issued Apr. 13, 1993, describes a method for anisotropically reactive ion etching aluminum and aluminum alloys. The plasma is comprised of a chlorinated and a carbon-containing gas mixture wherein the chlorinated gas provides the etching and the carbon-containing gas reacts to provide an inhibiting layer along the side wall of the aluminum layer. The plasma gas mixture is such that the ratio of carbon atoms to chlorine molecules results in an unexpected increase in the etch rate of the aluminum.
U.S. Pat. No. 5,158,644 to Cheung et al., issued Oct. 27, 1992, discloses a reactor chamber self cleaning process recommended for CVD (chemical vapor deposition) and PECVD (plasma-enhanced chemical vapor deposition) process chambers. The cleaning process can be used for both wide area cleaning of the chamber components and exhaust system components, as well as for local cleaning of the gas distribution manifold and RF electrodes. The self cleaning can be conducted when the process chamber is empty or may be used as an integral step during the processing of a substrate in particular cases. In particular, a process is described wherein dielectric coatings such as silicon oxide are deposited on a semiconductor wafer, after which self-cleaning is carried out in the reactor with the semiconductor wafer still present in the reactor. Obviously, a semiconductor wafer surface composition other than silicon oxide, which would be harmed by the self-cleaning process, cannot be left in the process chamber during the self-cleaning process.
When wide area cleaning is carried out, typical process conditions used for a CVD process chamber having a volume of approximately 5.5 gal. (about 21 liters) include use of C.sub.2 F.sub.6 gas at a flow rate of about 300 to 1,200 sccm, O.sub.2 gas at a flow rate of about 400-950 sccm, pressure of about 0.8 to 2 Torr, electrode spacing of about 1,000 mils and RF power density of about 2.7-5.6 watts/cm.sup.2. When local area cleaning is carried out, typical process conditions include a C.sub.2 F.sub.6 flow rate of about 600-950 sccm, an O.sub.2 flow rate of about 700-1,000 sccm, electrode spacing of about 180-350 mils, pressure of about 6-13 torr and a power density of about 2.7-5.6 watts/cm.sup.2.
U.S. Pat. No. 5,085,727 to R. J. Steger, issued Feb. 4, 1992, discloses an improved plasma etching apparatus comprising an etch chamber having inner metal surfaces coated with a conductive coating capable of protecting such inner metal surfaces from chemical attack by reactant gases such as halogen-containing gases used in the chamber during the plasma etching processes. In a preferred embodiment, a carbon coating at least about 0.2 micrometers is formed on the inner metal surfaces of the etch chamber by a plasma assisted CVD process using a gaseous source of carbon and either hydrogen or nitrogen or both. The conductive coating material is said to comprise a material selected from the group consisting of carbon, titanium nitride, indium stannate, silicon carbide, titanium carbide and tantalum carbide.
U.S. Pat. No. 4,786,359 to Stark et al., issued Nov. 22, 1988, describes a plasma etch process and apparatus in which silicon wafers are etched using a plasma-assisted gas mixture comprising CF.sub.3 Br and xenon or krypton. The use of CF.sub.3 Br is said to cause the deposition of polymeric material in the plasma reactor. The polymer formation is said to change the electrical characteristics of the chamber as well as the chemistry of the process being performed therein. Further, the polymer coating formed is said to become a source of particle contamination on the wafer. To solve this problem, Stark et al. added, within the chamber, a sacrificial structure which erodes during the etch process to prevent polymer buildup in the reactor chamber. The sacrificial structure is described as a carbon bearing object. In particular, the carbon bearing material is said to be an organic compound or graphite. High temperature plastics are said to appear to be suitable as carbon-bearing materials; of the high temperature plastics, polyarylates are said to etch more quickly than polyimides so that polyimides are preferred for use. Graphite, a graphite compound or a graphite-coated ceramic are given as preferred carbon-bearing materials.
Descriptions of the interrelationship between plasma etching and plasma polymerization, with emphasis on the plasma-surface interactions leading to polymerization are presented in "Plasma Polymerization of Fluorocarbons in RF Capacitively Coupled Diode System" by E. Kay and A. Dilks, J. Vac. Sci. Technol. 18 (1) January/February 1981. Further description of the use of fluorine and chlorine containing gases in plasma etching is provided in "Today's Plasma Etch Chemistries", Peter H. Singer, Associate Editor, Semiconductor International, March 1988. These articles make it clear that the development of a successful etch chemistry requires a careful selection of input gas composition as well as careful control of the process variables, including gas flow rate, chamber pressure and temperature, plasma energy and system configuration. Typically, the etch process must be tailored to the particular material to be etched (with process parameters being adjusted within predictable ranges in view of the particular system configuration).
Some of the U.S. patents referred to above describe the "dry" cleaning of semiconductor process chambers using plasmas. Other patents and the papers cited above describe the use of carbon-containing materials in gas plasma reactions to: polymerize on the side walls of etched aluminum under glow discharge conditions, preventing the undercutting of aluminum side walls during aluminum plasma etching; to prevent the build up of polymeric materials on the wall of the plasma chamber during the etching of silicon wafers; and provide a carbon coating on the walls of plasma chambers which protects the chamber walls from attack by halogen-containing gases during plasma etching processes. Although the functional behavior of the carbon-containing materials appears to be somewhat inconsistent in view of the descriptions provided in the patents, it is readily apparent that the carbon-containing materials react under plasma glow discharge conditions to form various chemical compounds such as polymers. These chemical compounds affect other process variables within the plasma-assisted process being carried out within the semiconductor process chamber.
As previously described, there is an interest in reducing the amount of time required for plasma cleaning of reactor chambers. The build up of contaminant deposits on the walls of plasma process chambers occurs to some extent during most plasma processes, but is particularly acute in metal etch processes. During chlorine-based metal etching of aluminum, for example, the aluminum reacts with chlorine molecules and atoms to form volatile aluminum chloride; some of this metal etch byproduct is pumped out of the plasma process chamber by applied vacuum. However, some of the chlorine species react with organic species from patterning photoresist and/or other organic sources within the reactor to form non-volatile materials which are deposited on the walls of the plasma process chamber. As increasing numbers of substrates are processed, the contaminating deposits on the process chamber wall increase in thickness and eventually begin to flake off due to thermal expansion and contraction of the processing equipment, and in some cases due to reactions with moisture; this flaking off of contaminants leads to particulate contamination of substrates being processed within the chamber.
The contaminating deposits on plasma process chamber walls can be removed in a plasma either by ion bombardment or by chemical reaction. Since the plasma chamber wall is normally electrically grounded, the ion bombardment (sputtering effect) upon the chamber wall itself is generally not very effective, and chemical reaction is preferred for cleaning process chamber surfaces. The most preferred way to remove the contaminant deposits using a chemical reaction is to convert the deposits to a volatile species which can be vacuum pumped from the plasma process chamber. Thus, it is desired to provide a method of dry cleaning plasma process chambers, particularly metal etch chambers, which converts contaminant deposits on the surfaces of the process chamber to volatile species which can be easily removed from the process chamber.